Recently, studies have been actively made for applications of closed vesicles such as liposome, emulsion, and lipid microsphere as drug carriers have been activity made for drug delivery system (hereinafter referred to as DDS). These closed vesicles are usually prepared using phospholipids or their derivatives, sterols, or lipids except phospholipids as basic membrane-constituting materials. However, the closed vesicles composed of only these basic materials could not overcome various problems occurring in the practical use, such as mutual aggregation of closed vesicles and shortened retention time in circulation. Furthermore, it was practically difficult for them to drive drugs to their target sites.
In order to improve a drug-loading and cell binding ability of the closed vesicles, an attempt has been made to charge the surface of the closed vesicles to cationic at physiological pH range by charging a small amount of a cationic lipid such as stearylamine. In particular, cationic liposomes containing DNA are well known to promote transfer of the DNA into cells, that is, transfection, and those liposomes having higher introduction efficiency, higher expression level, and higher safety level are strongly required. But, there are limited kinds of lipid available to use for cationic liposome, then it has been desired to develop cationic lipids usable for a drug carrier that are highly safe and exhibit high performance. Such cationic lipids have been reported so far in U.S. Pat. Nos. 4,897,355, 5,334,761, JP-A 2-292246, and JP-A 4-108391. However, their effects are not satisfactory.
Therefore, it has been eagerly desired to develop a drug carrier that can transfect a DNA into cells efficiently and safely. And regarding other purpose in addition to the transfection of a DNA, the effective and safety carriers are also required for effective DDS therein to enable accurate, efficient, and safe targeting of a drug to such as injured sites in vascular endothelium and affected sites caused by nephritis, kidney cancer, pneumonia, lung cancer, hepatitis, hepatoma, pancreatic cancer, lymphoma, etc.